1. Field of the Invention
The present invention relates to an ATM (Asynchronous Transfer Mode) multiplexing apparatus, and particularly, to an ATM multiplexing apparatus for performing cell discard processing on the basis of a communication state of the connection between ATM cells from the side of an ATM switching unit and subscribers.
2. Description of Related Art
An ATM network communication system has become popular as service for integrally conducting data communication including voice and text data, further image data requiring high-speed transmission. In the ATM network communication system, cell discard processing is performed in order to protect an ATM transmission band from an ATM cell caused by abnormality etc. of subscriber terminals. The cell discard processing is carried out mainly by an ATM switching unit and an ATM multiplexing apparatus.
With regard to public known arts related to the present invention, technique on a control method of monitoring the packet flow rate is disclosed in Japanese Unexamined Patent Publication No. 46344 of 1997. This control method provides a plurality of usage parameter control mechanisms at the entrance of an ATM network. In each of the usage parameter control mechanism, the transfer rate defined in ABR (Available Bit Rate) communication service is set as a monitor parameter. Thus, monitor operations according to the use state of the ATM network are executed. Also, technique on an ATM communication network comprising ATM switching units connected in a multistage manner is disclosed in Japanese Unexamined Patent Publication No. 261244 of 1997. In this ATM communication network, if congestion has occurred in any of the ATM switching units, an ATM switch of the fore-stage of the ATM switching unit in which the congestion has occurred is notified of the congestion. The notified ATM switching unit controls the discard state of cells. Further, technique on a packet control switching unit and a cell transfer control method are disclosed in Japanese Unexamined Patent Publication No. 13427 of 1998. In this cell transfer control method, the ATM switching unit acting as a node unit stores priority on cell discard corresponding to a connection identifier. Cell discard processing is controlled on the basis of discard conditions determined by the relationship of the stored cell priority to the number of stack cells in the node every connection.
FIG. 11 shows a conceptual diagram of a conventional ATM network system. An ATM switching unit 201 is installed in a central station. ATM multiplexing apparatus 301 and 303 are largely installed at remote sites nearest to subscribers. The ATM multiplexing apparatus 301 and 303 desirably have a cheap and simple configuration where possible.
Functions necessary for the ATM multiplexing apparatus 301 and 303 include a UPC (Usage Parameter Control) function. An object of the UPC function is to prevent congestion in an ATM network by checking whether the ATM cell flow from the subscribers violates a contract band or not. In order to efficiently share a band of an ATM transmission line with all the subscriber signals, the UPC to input signals of each subscriber line before multiplexing needs to be performed. However, installation of this UPC units 309 and 311 increases the cost per subscriber line. On the other hand, when SVC (Switched Virtual Connection) service for performing dynamic signal setting is adopted, traffic setting information set by signaling needs to be passed between the ATM switching unit and the ATM multiplexing apparatus. Passing of the traffic setting information can be managed by installation of a communication line for management, but the amount of processing of the ATM multiplexing apparatus increases.
In the conventional ATM network system as shown in FIG. 11, the ATM multiplexing apparatus 301 provides the UPC units 309 every subscriber line inside the units, so that the cost of installation increases. The UPC unit 311 is installed inside of the ATM multiplexing apparatus 303 to monitor a signal after multiplexing in a lump, so that the cost of installation of the UPC unit 311 decreases. Even in the case of installing any of the ATM multiplexing apparatus 301 and 303, traffic information from a traffic processing control part 211 is transferred from the ATM switching unit 201 to the ATM multiplexing apparatus 301 and 303 through traffic information transfer means 213. The traffic information transfer means 213 means communication between respective unit control parts. Coping with the traffic information transfer means 213 requires the increase in the cost of the ATM multiplexing apparatus 301 and 303.
Under such circumstances, the ATM multiplexing apparatus and the selective cell discard method for performing cell discard processing efficiently in the case of congestion on the basis of a use state of the connection formed by ATM cells received from the side of the ATM switching unit and ATM cells received from the side of the subscribers without installing the UPC units have been desired. It is desirable to detect a congestion state corresponding to the ATM cell flow sent from each subscriber. It is desirable to sequentially update the use state of the connection on the basis of available information on the ATM cells received from the ATM switching unit or the subscribers. The discard processing is desirably performed on the basis of a logical decision using a plurality of data providing the use state (communication state) of the connection and level information of the congestion state.